Phase Diagrams and Functional Properties of Transition-Metal-Alloy Clusters

在本研究中，我们阐明了以下几点：1）我们在等离子体气体冷凝团簇沉积装置中安装了两种金属靶，制备了直径为5-10 nm的合金团簇，并沉积在基底上。透射电子显微镜观察和EDX分析表明与平衡相图相比的特征方面：（a）Co-Al中的成分起伏不明显，形成B2型有序相（B）在Co-Pt合金中，成分波动很小，存在无序的fcc相（c）Co-Pd合金中的组分波动增强，形成了fcc结构的无序相（均匀固溶体型），和（d）组成分布曲线具有双峰，但主要固溶体形成在Nb-Ag中（不混溶型）。2）290 K下的磁化曲线随团簇尺寸的变化显示在直径约8 nm处发生铁磁-超顺磁体转变。3）Co核壳团簇组装体，Co CSCA，通过轻微的氧化和沉积在衬底上制备的，显示出显着的隧道型导电性和具有库仑阻塞效应的磁电阻。电阻率测量表明，随着氧化程度的增加，Co、Ti和Cr CSCA从金属（电阻率温度系数TCR为正）转变为半导体（TCR为负：激活型），而Cu CSCA始终为金属。在中间氧化阶段，TCR在高温下为正，而在低温下变为负（-logT依赖性），表明弱的电子局域化效应。我们正在研究它们的电子状态。这些核壳结构的团簇将用于微型气体传感器和催化剂。

In this research, we have elucidated the following points1) We have installed two sorts of metal targets in the plasma-gas-condensation cluster deposition apparatus, produced alloy clusters with 5-10 nm in diameter, and deposited on a substrate. The transmission electron microscope observation and EDX analysis indicate characteristic aspects in comparison with the equilibrium phase diagrams : (a) the compositional fluctuation is not so marked and a B2-type ordered phase is formed in Co-Al (a compound formation type), (b) the compositional fluctuation is slightly detectable and a disordered fcc phase in Co-Pt (an ordered phase formation type), (c) the compositional fluctuation is enhanced and a fcc disordered phase in Co-Pd (a homogeneous solid solution type), and (d) the compositional distribution curve has double peaks, but the primary solid solutions are formed in Nb-Ag (an immiscible type).2) Magnetization curves at 290 K as a function of the cluster size show ferromagnet-superparamagnet transition at around 8 nm in diameter.3) Co core-shell cluster assemblies, Co CSCA, prepared by slight oxidation and deposition on a substrate, show a marked tunneling type conductivity and magnetoresistance with a Coulomb-blockade effect. It also shows a crossover from a thermal type to a quantum tunneling type magnetic relaxation.4) The electrical resistivity measurement indicates that with increasing the degree of oxidation, Co, Ti and Cr CSCA changes from a metal (the temperature coefficient of resistivity, TCR, is positive) to a semiconductor (TCR is negative : an activation type), while Cu CSCA is always metallic. In the intemediate oxidation stage, TCR is positive at high temperature, while it become negative (-logT dependence) at low temperature, indicating a weak electron locarization effect. We are investigating their electronic states. These core-shell clusters will be useful for a tiny gas sensor and catalyses.